1. Field of the Invention
The present invention relates to a printing apparatus and processing method thereof.
2. Description of the Related Art
There is known a printing apparatus which employs an inkjet method of printing an image on a printing medium using a printhead including orifice arrays each configured by arraying a plurality of printing elements (orifices) (integrating and arraying many printing elements). As printing apparatuses of this type require higher printing operation speeds and higher resolutions, the number of orifices arrayed on a printhead is increasing.
When all printing elements are simultaneously driven in a printing operation, discharge becomes unstable owing to pressure interference (crosstalk) between neighboring orifices, and the like. Since a large current is supplied, a voltage drop arising from power loss on a common power line becomes large near the printhead. As the number of simultaneously driven orifices increases, driving voltage applied to orifices (printing elements) drops more steeply, impairing the printing stability. Further, a power supply instantaneously resistant to a large current is necessary, inhibiting the design of a compact, low-cost apparatus.
To solve these problems, all orifices are generally divided into a plurality of driving blocks in a printhead, and orifices in the respective driving blocks are time-divisionally driven sequentially. This driving method is called time divisional driving (or block divisional driving).
When a printhead in which printing elements are arranged on a single straight line is time-divisionally driven for respective driving blocks, the printing position shifts between the driving blocks because the printhead moves in the scanning direction during the time divisional driving. For example, when expressing tonality using a unit matrix (an image processing control unit formed from M×N pixels), a dot pattern in the matrix may shift in every printing scan of the printhead in accordance with the relationship between the matrix size and the pattern size of the driving block. To solve this problem, Japanese Patent Laid-Open No. 2006-159698 proposes a method of shifting the arrangement of binary image data in every printing scan of the printhead in accordance with the relationship between the matrix size and the pattern size of the driving block.
Conventional printing by time divisional driving suffers the following problem regardless of whether to express tonality using a unit matrix.
FIG. 13 is a view showing the relationship between the orifice array of a printhead, the driving signal of each orifice, and a dot which is discharged from each orifice and attached to a printing medium. FIG. 13 shows 2-pass printing (that is, in which an image is printed by two printing scans) in the same printing region on a printing medium.
In this case, every time a printing scan is performed, the printing medium is conveyed by a distance corresponding to eight orifices. Reference numeral 401 denotes a first printing scan; 402 and 412, a second printing scan; and 403 and 413, a third printing scan.
An orifice array denoted by reference numeral 402 is illustrated at a position shifted from an orifice array denoted by reference numeral 401 by eight orifices in the orifice array direction (printing medium conveyance direction). This is because in the second printing scan, the printing medium is conveyed in the conveyance direction by a distance corresponding to eight orifices from a position in the first printing scan. Similarly, an orifice array denoted by reference numeral 403 is illustrated at a position shifted along with conveyance of the printing medium.
An orifice array 500 of the printhead is divided into two, groups 1 and 2 each including eight adjacent orifices, as denoted by reference numerals 401 to 403. Each of eight orifices in each group belongs to one of eight driving blocks. In a printing operation, the eight orifices are time-divisionally driven for the respective driving blocks (orifices of the same driving block are driven simultaneously). Note that numerals on the left side of respective orifices indicate orifice numbers 1-1 to 2-8, and numerals on the right side of respective orifices indicate block numbers 1 to 8.
In the orifice array 500, the first and ninth orifices 1-1 and 2-1 from the top in FIG. 13 are assigned to the first driving block. The second and 10th orifices 1-2 and 2-2 from the top in FIG. 13 are assigned to the second driving block. All orifices are assigned to driving blocks. The first to eighth driving blocks are sequentially driven in the ascending order based on a pulse-like block selection signal 300 as denoted by reference numerals 411 to 413, and a printing signal complying with image data. Then, ink is discharged from the respective orifices, forming dots on a printing medium, as denoted by reference numeral 414.
As the layout positions of dots formed on a printing medium, dots are formed in a staggered pattern in the first scan (first scanning), and dots are formed in an inverse staggered pattern in the second printing scan (second scanning) in the same printing region, as denoted by reference numeral 414. Printing of an image is completed by 2-pass printing.
To the contrary, FIG. 14 shows printing using only a predetermined number (eight in this case) of orifices positioned at the center, unlike printing using all orifices in FIG. 13. For example, an orifice array denoted by reference numeral 601 prints using orifices 1-5 to 1-8 and 2-1 to 2-4. Note that the arrangement of the printhead and original image data are the same as those in FIG. 13.
A comparison between dot layout positions denoted by reference numeral 414 in FIG. 13 and those denoted by reference numeral 616 in FIG. 14 reveals that they are different from each other, though original image data is the same.
More specifically, dots are laid out with almost no gap on a printing medium at dot layout positions denoted by reference numeral 414 in FIG. 13. In contrast, gaps are generated between dots at dot layout positions denoted by reference numeral 616 in FIG. 14. This dot layout position difference is generated because all dots are formed by the same driving blocks in FIG. 13, whereas dots formed by different driving blocks coexist in FIG. 14.
When a region printed using all orifices and a region printed using only some orifices exist, the relationship between the printing medium conveyance amount and the driving block cycle changes, and fill of dots differs between the respective regions. This appears as density nonuniformity, degrading image uniformity.